Disc drive storage devices typically store binary data onto the surface of a rotating disc in divisible units referred to as tracks, where each track is divided into a number of data units referred to as sectors. In magnetic storage devices, for example, the digital data serves to modulate a write current in a inductive recording head in order to write a series of magnetic flux transitions onto the surface of a magnetizable disc in a series of concentric, radially spaced tracks. And in optical recording systems, the digital data may modulate the intensity of a laser beam in order to record a series of "pits" onto the surface of an optical disk in spiral tracks.
The host system connected to the storage device accesses the disc drive by writing and reading data to and from a particular sector. The disc drive positions a recording head (or transducer) over the track containing the requested sector, waits for the disc to rotate until the recording head is over the requested sector within the track, and then performs a write or read operation on the sector. The latency associated with spinning the disc to the requested sector is a significant factor in the overall operation speed (access time) of the disc drive. Once the transducer reaches the target track, the storage system must wait for the disc to complete one-half a revolution on average to reach the target sector for every read and write operation requested.
The sectors on a track typically include user data and appended sector level redundancy symbols for detecting and correcting errors in the user data when reading the sector from the disc. During a read operation, a sector level error correction system (ECS) uses the sector level redundancy symbols to detect and correct errors in the user data that occur due, for example, to noise or defects in the recording/reproduction process. If the number of errors detected exceeds the error correction capability of the sector level ECS, then depending on the nature of the errors, the entire sector may be unrecoverable. Random errors caused by noise in the reproduction process (e.g., electronic noise induced in the read signal) are referred to as "soft errors" because they may not necessarily render the sector permanently unrecoverable. That is, the storage system can "retry" the read operation until the number of soft errors is within the error correction capability of the sector level ECS.
Permanent errors, or "hard errors", are typically associated with defects (drop-outs, aberrations, etc.) on the surface of the disc which render the medium permanently unrecoverable if the number of hard errors exceeds the error correction capability of the sector level ECS. Further, every sector typically includes a preamble field and a sync mark for use by timing recovery in synchronizing to the data in the sector. If a hard error corrupts this timing information, then the entire sector may become completely unreadable due to the inability to synchronize to the data.
In the context of this application, an unrecoverable sector refers either to a readable but uncorrectable sector at the sector level, or an unreadable sector due, for example, to an inability to synchronize to the sector data.
There are prior art disc storage systems which attempt to protect against losing an entire sector that has become unrecoverable at the sector level. For example, U.S. Pat. No. 5,392,290 entitled "System and Method for Preventing Direct Access Data Storage System Data Loss from Mechanical Shock During Write Operation," suggests using a parity sector within each track, wherein the parity sector comprises the XOR (parity) of all of the data sectors for that track. In this manner, if any one of the data sectors becomes unrecoverable, it can be completely reconstructed using the parity sector.
The parity sector in the above scheme is updated during each write operation by first reading the sector that is to be over written and "backing out" its contribution to the parity sector (by XORing it with the parity sector). Then, the new sector is written to the disc and added (XORed) into the parity sector. The updated parity sector is then written back to the disc. If a particular sector is determined unrecoverable during a read operation, then to recover that sector the storage system reads and XORs the other sectors in the track (including the parity sector), and the result of the XOR operation is the unrecoverable sector.
This track level parity sector scheme for recovering an unrecoverable sector has not been widely employed in disc storage systems due to the intolerable increase in latency associated with updating the parity sector during each write operation. That is, the storage system must seek to the sector to be over written, read that sector (or sectors), and "back out" its contribution to the parity sector. Then, it must wait for a complete revolution in order to write the new sector (or sectors). Finally, the storage system must wait for the disc to spin to the parity sector so that it can over write it with the updated parity sector. Further, the revolution of latency associated with backing out the contribution of the target data sectors from the redundancy sector applies even if the write range spans one less sector than the entire track.
Another problem inherent in the prior art track level parity sector scheme is that it can correct only one unrecoverable sector per track. Thus, if two or more sectors on a track become unrecoverable, the prior art parity sector scheme is rendered useless.
Yet another problem not addressed by the prior art parity sector scheme is that a sector can become unrecoverable due to errors associated with a write operation on that sector. For example, a defect on the medium may result in a hard error depending on how the sector data is written to the disc. That is, a corrupted write operation may result in excessive hard errors which render the sector uncorrectable, whereas another write operation may not. For example, a phenomena that can result in an unrecoverable sector, known as "high write", occurs when an anomaly on the medium causes the fly height of the recording head to increase, thereby decreasing the magnetization strength of the inductive write signal. Thus, if a first sector on a track becomes unrecoverable due to a corrupted write operation, and no attempt is made to read that first sector before a second sector becomes unrecoverable due to a subsequent corrupted write operation, then the prior art parity sector scheme will be unable to recover either sector.
Consequently, most disc storage systems do not employ a track level parity sector; instead, they take other precautions to protect against influences which may render a sector unrecoverable. Namely, to protect against hard errors which may render a sector unreadable due to defects in the medium at the preamble or sync mark fields, the entire disc is tested during manufacturing. If it is determined that the preamble or sync mark field cannot be read due to defects in the medium, then that sector is mapped to a spare sector. A similar "defect scan" and "defect mapping" can be performed for the entire sector to determine if the number of resulting hard errors will exceed the error correction capability of the sector level ECS. Alternatively, a system designer may increase the error correction capability of the sector level ECS to decrease the probability that a sector will become uncorrectable.
The problem with scanning the medium for defects during the manufacturing process and mapping bad sectors to spare sectors is that it does not account for "grown defects", defects that arise during the lifetime of the storage system. Grown defects include, for example, invading foreign particles which become embedded onto the surface of the disc, or external shocks to the storage system which can cause the transducer to nick the surface of the disc. Furthermore, there are problems associated with increasing the error correction capability of the sector level ECS to overcome grown defects. Namely, it becomes prohibitively complex and expensive to implement, and it reduces the capacity of the storage system due to the increase in the sector level redundancy bytes.
There is, therefore, a need for a disc storage system that can protect against read errors rendering a sector unrecoverable, without increasing the cost and complexity of the sector level ECS, and without the above mentioned problems associated with the prior art track level ECC scheme.